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{-# LANGUAGE DeriveDataTypeable #-}
{-# LANGUAGE DeriveGeneric #-}
{-# LANGUAGE EmptyCase #-}
{-# LANGUAGE EmptyDataDeriving #-}
{-# LANGUAGE Safe #-}
{-# LANGUAGE StandaloneDeriving #-}
-----------------------------------------------------------------------------
-- |
-- Copyright : (C) 2008-2014 Edward Kmett
-- License : BSD-style (see the file libraries/base/LICENSE)
--
-- Maintainer : Edward Kmett <ekmett@gmail.com>
-- Stability : provisional
-- Portability : portable
--
-- A logically uninhabited data type, used to indicate that a given
-- term should not exist.
--
-- @since 4.8.0.0
----------------------------------------------------------------------------
module Data.Void
( Void
, absurd
, vacuous
) where
import Control.Exception
import Data.Data
import Data.Ix
import GHC.Generics
import Data.Semigroup (Semigroup(..), stimesIdempotent)
-- | Uninhabited data type
--
-- @since 4.8.0.0
data Void deriving
( Eq -- ^ @since 4.8.0.0
, Data -- ^ @since 4.8.0.0
, Generic -- ^ @since 4.8.0.0
, Ord -- ^ @since 4.8.0.0
, Read -- ^ Reading a 'Void' value is always a parse error, considering
-- 'Void' as a data type with no constructors.
--
-- @since 4.8.0.0
, Show -- ^ @since 4.8.0.0
)
-- | @since 4.8.0.0
instance Ix Void where
range _ = []
index _ = absurd
inRange _ = absurd
rangeSize _ = 0
-- | @since 4.8.0.0
instance Exception Void
-- | @since 4.9.0.0
instance Semigroup Void where
a <> _ = a
stimes = stimesIdempotent
-- | Since 'Void' values logically don't exist, this witnesses the
-- logical reasoning tool of \"ex falso quodlibet\".
--
-- >>> let x :: Either Void Int; x = Right 5
-- >>> :{
-- case x of
-- Right r -> r
-- Left l -> absurd l
-- :}
-- 5
--
-- @since 4.8.0.0
absurd :: Void -> a
absurd a = case a of {}
-- | If 'Void' is uninhabited then any 'Functor' that holds only
-- values of type 'Void' is holding no values.
--
-- Using @ApplicativeDo@: \'@'vacuous' theVoid@\' can be understood as the
-- @do@ expression
--
-- @
-- do void <- theVoid
-- pure (absurd void)
-- @
--
-- with an inferred @Functor@ constraint.
--
-- @since 4.8.0.0
vacuous :: Functor f => f Void -> f a
vacuous = fmap absurd
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